Dolomite (pron.: /ˈdɒləmaɪt/) is a carbonate mineral composed of calcium magnesium carbonate CaMg(CO3)2.
Dolostone (dolomite rock) is composed predominantly of the mineral dolomite with a stoichiometric ratio of 50% or greater content of magnesium replacing calcium, often as a result of diagenesis.
Dolomite is used as an ornamental stone, a concrete aggregate, a source of magnesium oxide and in the Pidgeon process for the production of magnesium. It is an important petroleum reservoir rock, and serves as the host rock for large strata-bound Mississippi Valley-Type (MVT) ore deposits of base metals such as lead, zinc, and copper. Where calcite limestone is uncommon or too costly, dolomite is sometimes used in its place as a flux for the smelting of iron and steel. Large quantities of processed dolomite are used in the production of float glass.
In horticulture, dolomite and dolomitic limestone are added to soils and soilless potting mixes to lower their acidity and as a magnesium source. Home and container gardening are common examples of this use.
Dolomite is also used as the substrate in marine (saltwater) aquariums to help buffer changes in pH of the water.
Particle physics researchers prefer to build particle detectors under layers of dolomite to enable the detectors to detect the highest possible number of exotic particles. Because dolomite contains relatively minor quantities of radioactive materials, it can insulate against interference from cosmic rays without adding to background radiation levels.
Dolomite can be used in glazes melting over 1170C to produce a silky matte surface. This occurs because high percentages of dolomite help to form diopside crystals (CaMg(SiO3)2) on cooling, and it is these that produce the popular butter-matte effect. This effect is most pronounced in reduction.
Dolomite as a ceramic material is a uniform calcium magnesium carbonate. In ceramic glazes it is used as a source of magnesia and calcia. Other than talc, dolomite is the principle source of MgO in high temperature raw glazes. ‘Dolomite matte’ stoneware glazes, for example, are highly prized for their pleasant ‘silky’ surface texture. Dolomite by itself is refractory, but when combined with the typical oxides in a glaze (especially boron) it readily enters the melt.
Commercial dolomites are not able to achieve the theoretical 54:46 calcium carbonate:magnesium carbonate ratio, they tend to have less magnesia. It is simple to do an LOI test by firing a specimen of powder in a thin bisqued bowl to confirm the consistency of dolomite shipments. The chemistry shown here is theoretical and many commercial materials approach this with much less than 1% of two or three other oxides (e.g. Al2O3, SiO2).
Dolomite is a carbonate (like whiting) in that it loses considerable weight during firing when it disassociates to form MgO, CaO and CO2, this process being complete by about 900C. However, in many circumstances where a raw glaze employs both CaO and MgO, dolomite is an economic alternative to sourcing with a mix of calcium carbonate and talc. However care needs to be taken to obtain a consistent grade since dolomites tend to vary more in mineralogy and can contain iron contamination that can darken the fired glaze.
Synthetic substitutes to source MgO and CaO (e.g. frits) are worth considering, especially if glazes are not high temperature. Frits have no loss on ignition (therefore do not generate glaze bubbles) and melt far earlier than mineral sources of MgO and CaO. Using ceramic calculations it is quite easy to adjust a recipe to source MgO from a frit instead of raw materials.